1. Field of the Invention
The present invention relates to a non-volatile memory device and a method for manufacturing such devices and, more particularly, to a non-volatile memory device having a dielectric film exhibiting high permitivity and superior electric properties produced according to the disclosed manufacturing method.
2. Description of the Related Art
In general, semiconductor memory devices can be divided into two main types according to the manner in which they store. The basis of this division is that volatile memory devices, such as DRAM (dynamic random access memory), must periodically restore the data and non-volatile memory devices such as SRAMs (static random access memory) and flash memories, do not need to periodically restore the data.
These two basic types of memory devices have different advantages and disadvantages that tend to make one type more suitable for a particular application. Engineers and designers typically evaluate the product needs and select the appropriate type of memory best suited for the particular application. In the case of flash memory devices, a type of non-volatile memory device, the demand is gradually increasing with the expanding market for portable electronic products. A dielectric film is used in the cell transistors of a flash memory device to insulate both a floating gate and a control gate. This dielectric film simultaneously functions as a dielectric layer in the cell capacitors of the flash memory.
A conventional ONO (oxide film/nitride film/oxide film) dielectric thin film, although commonly used as a dielectric film in flash memory devices, does not provide sufficient capacitance for operating the next generation flash memory products. In particular, it is difficult to reduce the thickness of an oxide film grown on heavily doped polysilicon using a thermal oxidation method without suffering a deterioration in the electrical properties. This deterioration is due to the high phosphorous (P) concentration in the floating gate, a high defective density, and irregularity and non-uniformity in the oxide produced by oxidizing the heavily doped polysilicon. Each of these factors lowers the effective capacitance, thereby preventing sufficient capacitance from being obtained.
Thus, a Ta2O5 thin film which may be broadly applied to DRAM products over 256M bytes may also be widely utilized as a dielectric film in flash memory devices. However, because the Ta2O5 film has an unstable stoichiometry, substitution type tantalum atoms (vacancy atoms) are generated as a result of differences in the combination ratio between tantalum (Ta) and oxygen (O) existing in the deposited thin film. In the Ta2O5 film, the substitution type tantalum atoms in an oxygen vacancy state always exist as a result of the unstable composition of the film itself. Thus, to compensate for the naturally unstable stoichiometry of Ta2O5 and prevent leakage current, a separate post-deposition oxidation process to more completely oxidize the substitution type tantalum atoms is required.
Also impurities such as, carbon (C) atoms, carbon compounds (CH4 and C2H4) and water (H2O) are created during the formation of the thin film as a result of reactions between the organometallic Ta(OC2H5) precursor to the Ta2O5 film and O2 (or N2O) gases. These impurities are, in turn, incorporated into the film, increasing leakage current generated from the floating gate of a cell transistor through the dielectric film, thereby deteriorating the dielectric properties.
For these reasons, Ta2O5 thin films are not generally applicable as the dielectric film for a cell transistor in flash memory devices.